Optical modulators have the ability to provide a reliable, cost-effective alternative to laser sources in remote terminals of optical communication networks. For example, modulators are particularly attractive in wavelength-division-multiplexed (WDM) systems, where strict requirements on frequency stability generally make laser-based remote terminals impractically expensive. In modulator-based networks, signals are generated by a light source (usually a laser) at a host terminal and distributed to remote terminals by a power splitter, wavelength router, or other means. These signals are then modulated at the remote terminals and returned to the host terminal either via the same fiber (bi-directional or duplex operation) or via a second, parallel fiber (unidirectional or simplex operation). A disadvantage of modulator-based networks is that transmission losses appear twice in the round-trip signal path, thus leading to limited power budgets for broadband systems.
Because cost targets are extremely stringent for remote terminals in local access networks, modulator packaging must minimize critical fiber alignments. Thus, single-port (reflection-mode) devices are generally preferred over two-port (transmission-mode) devices, wherein these devices should also be tolerant of fiber misalignment.
A number of modulator options exist in the prior art. Micro-mechanical modulators, for example, as described by Goossen et al. in "Silicon Modulator Based on Mechanically-Active Layer with 1 Mb/sec Capability for Fiber-in-the-Loop Applications," IEEE Photonics Technology Lett., vol. 6, pp. 1119-1121, 1994, are single-port, reflective modulators with an acceptable tolerance of fiber misalignment, and offer prospects for the narrowbad applications, but are limited in general to bitrates of a few Mb/s. Semiconductor Stark-effect modulators have better modulation bandwidth and can be operated in a single-port, normal-incidence package, but they can be undesirably wavelength- and temperature-sensitive. Conventional waveguide modulators in LiNbO3 or semiconductors have high modulation bandwidth, but require expensive two-port packaging with two independent fiber alignments made to single-mode tolerances. High bandwidth modulators with gain have been made by electrically switching semiconductor travelling-wave amplifiers as described by Koren et at. in "High Frequency Modulation of Strained Layer Multiple Quantum Well Optical Amplifiers," Electronics Lett., vol. 27, pp. 62-64, 1991, and Giliner in "Modulation Properties of a Near Travelling-Wave Semiconductor Laser Amplifier," IEE Proceedings-J, vol. 139, pp. 331-338, 1992, and the references cited therein. However, two-port packaging is still required with these devices. Accordingly, there is a need for a high performance optical modulator which provides gain without the high packaging costs associated with two-port devices.